1. Technical Field
The present invention relates to a transformer, more particularly to a high voltage transformer and a high voltage power supply apparatus having the same.
2. Description of the Related Art
A transformer is an apparatus for changing voltage value or current value of AC (Alternating Current) by using electromagnetic phenomenon.
In case that the voltage is raised by using the transformer, particularly a DC/DC converter, the transformer may be saturated by the operating frequency and the voltage applied to the first coils.
The saturation of the transformer is a phenomenon that if the magnetic flux density is over the specific value, the magnetic flux density rarely increase despite the increase of the magnetizing force although the magnetic flux density generally increases when the magnetizing force increase. And the saturation phenomenon induces a harmonic frequency which shortens the durability of the transformer or cause vibration of the transformer. In addition, the saturated transformer loses the function as a transformer; thereby the required second voltage cannot be obtained.
One method to prevent this saturation phenomenon is to increase magnetizing inductance by increasing the turns of the first coil of the transformer. But when magnetizing inductance is increased, the turns of the second coil of the transformer should be increased corresponding to the turns of the first coil in order to maintain the same raising ratio of voltage.
After all, in order to prevent this saturation phenomenon together with the stable output, the minimum turns of the first coil for preventing saturation together with the ratio of the turns of the first coil and the second coil for maintaining the same raising ratio, and the insulation between the first coil and the second coil and the insulation between the layers in the second coil corresponding to the turns of the first coil, should be considered together.
FIG. 14 shows a structure of the prior transformer designed by the above method.
The transformer in FIG. 14, is provided with a core 110, first coil unit wound in the core 110 (not shown because first coil unit wound in the core 110 is wound within the bobbin), a bobbin 130 of insulating material such as Teflon, and second coil unit 140 wound on the bobbin 130. And the transformer is connected with the supporting unit by the connecting member such as bolt 170, and a bus bar 150 may be provided around the transformer.
FIG. 15 shows a side view of the prior transformer of FIG. 14.
The prior transformer generally uses a method in which the winding area is increased by increasing the length of the core in winding coil on the bobbin 130 for the second coil unit 140. That is to say, since the turns of the first coil unit is increased in order to prevent saturation of the transformer, and the turns of the second coil unit 140 is increased in proportional with the increase of the turns of the first coil unit, the size of the core should be increased in order to provide increased space for the increased turns of the second coil unit. But this method has a problem that the volume of the transformer and the manufacturing cost increase.
Generally, when the higher raising ratio in the transformer is required, more turns of the second coil than the turns of the first coil unit in the bobbin. In addition, in case that the turns of the first coil unit increase in order to prevent saturation of the transformer, the turns of the second coil unit also increase. Therefore, the coil of the second coil unit should be wound in two or more layers in the bobbin in case that it is impossible to wind coil of the second coil unit in one layer.
FIG. 16A shows the prior method for winding coil of the second coil on the bobbin in the transformer considering the layer insulation, and FIG. 16B shows a circuit diagram in a state of coil winding on the bobbin of the prior transformer.
But since the insulation between the layers in the second coil unit should be considered, the region enabling the winding of the coils of the second coil unit on the bobbin in the higher layer continuously decreases as shown in FIG. 16B.
According to the prior method for winding coil of the second coil in the prior transformer, coil is wound around the bobbin 130 from one end of the bobbin 130 in a direction of the arrow numbered 1. After reaching the other end of the bobbin 130 by being wound in the direction of the arrow numbered 1, the coil in the next layer is wound from the other end of the bobbin 130 in the direction of the arrow numbered 2.
And when reaching the one end of the bobbin 130 by being wound in the direction of the arrow numbered 2 in the next layer, the coil in the next layer will contact the first end of the coil in the first layer, and there is a great difference in voltage between the end of the coil in the next layer the direction of the arrow numbered 2 and the first end of the coil in the first layer.
Therefore, in order to maintain the insulation between layers in the first coil unit, the coil should be wound by remaining ‘A’ portion as in FIGS. 16A and 16B empty in the direction of the arrow numbered 3.
By repeating these processes, when reaching the one end of the bobbin 130 by being wound in the direction of the arrow numbered 3, the coil is again wound by remaining ‘B’ portion as in FIGS. 16A and 16B empty in the direction of the arrow numbered 4 considering the layers insulation.
But when the coil in the second coil unit is wound on the bobbin 130 by this method, the longitudinal center of the high voltage transformer has a convex shape, which results in increase in volume of the transformer and makes maintenance of a proper distance from the second coil unit of the adjacent transformer impossible.
In addition, there is a problem that the turns in the second coil unit are limited when the height of the coil layers becomes higher, and then efficiency falls.
In particular, when the second coil unit wound in the core of EE shape or UU shape, the distance between the second coils wound on both sides decreases, thereby insulation destruction phenomenon may occur.